1. Field of the Invention
The present invention relates to a composite memory device which, by including two kinds of memory, enables a reduction in access time. One kind of memory is a magnetic or optical disk which require a relatively long access time to read data recorded on a recording track of the disk or to write data on a recording track thereon, the other is semiconductor memory adapted to contain data for use in accessing files on the disk.
2. Description of the Prior Art
Magnetic or optical disks are formatted to enable data to be recorded thereon. FIG. 1 shows one example of the format of a magnetic recording medium or disk. The disk has several tracks 15, 16, each of which being divided into several sectors 13, 14. The tracks 15, 16 and the sectors 13, 14 are allocated track addresses and sector addresses, respectively, and are adapted to contain data. FIG. 2 shows one example of the relationship between tracks and sectors. The tracks are called track 0 (17), track 1 (18), track 2 (18) . . . in order from the outer periphery of the magnetic recording disk, track 0 (17) defining a directory area used to locate file data recorded on the magnetic recording disk, track 1 (18), track 2 (18) . . . after track 0 defining file areas. When data in a certain file area is to be read, a head of an external unit such as a disk drive unit (hereinafter referred to as a disk drive) is first moved to the directory area to read the track and sector addresses corresponding to the track of the data which is to be read, then the head is shifted to the corresponding file area to read the data. In the above-described arrangement, the time required for the disk drive to access the data is relatively long on average, up to about 100 milliseconds.